1. Field of the Invention
The present invention relates to a fluidized bed reactor having in its lower part a furnace section, delimited by side walls and a bottom grid, and supplying means, for introducing a gas, such as partial combustion air, into a bed of fluidized particles in the furnace section. Such supplying means may include a gas source chamber, such as a windbox, and at least one nozzle or conduit connected to a respective opening in the side wall, for introducing gas from the gas source chamber to the furnace section.
This invention is particularly applicable to large scale circulating fluidized bed (CFB) boilers having a thermal effect of, e.g., 200-400 MWe, or more, in which boilers the lower section of the boiler furnace and the bottom grid may, if desired, be divided in two or more furnace sections, e.g., by a dual wall partition structure. The dual wall partition structure may be a complete partition wall reaching in the furnace from one wall to the opposite wall or a partial wall, i.e., the dual wall construction may consist of a continuous or a discontinuous wall between two opposite furnace walls. The partition wall structure, which typically is of a dual wall construction, may be made by a refractory wall or a cooled wall connected to a cooling water circulation system of the boiler.
Accordingly, in the large scale boilers to which the present invention is applicable, the partial combustion air may be distributed through one or more gas source chambers connected to the external side walls and/or connected to the partition wall structure, if such a wall structure is utilized.
2. Related Background
Optimized emission control and maximum fuel burn-up are decisive qualifications for a successful furnace design. Thus, they must especially be taken into consideration in circulating fluidized bed scale-up. A simple proportional scaling up of designs used in smaller systems may easily lead to problems in attempting to provide for a good mixing of fuel, combustion air and fluidized bed solids. Additionally, such designs may suffer from not being capable of providing a uniform furnace temperature within the optimum range and a sufficient heat transfer area. All these problems, which may cause enhanced emissions and less than optimal fuel burn-up, have led to a desire to find alternative solutions. Such solutions have, e.g., included designs with multiple furnaces with a common back pass, providing heat transfer panels and/or partial or full division walls within the furnace, or dividing the lower part of the furnace and the bottom grid with, e.g., a dual wall structure.
Different solutions for sectioning the bottom area of a fluidized bed boiler furnace are known in the prior art. U.S. Pat. No. 4,864,944 discloses a division of a fluidized bed reactor into compartments by partition walls having openings for secondary gas to be distributed in a desired manner into the reactor. The partition walls have ducts which are connected to air supply sources and lead to discharge openings at different heights in the partition walls.
Correspondingly, U.S. Pat. No. 4,817,563 discloses a fluidized bed system provided with one or more displacement bodies, which may be provided with lines and inlet openings for introducing secondary gas to segmented sections in the lower reactor.
U.S. Pat. No. 5,370,084 discloses different configurations for effective mixing of fuel in a partitioned circulating fluidized bed boiler, including ducts which feed air into the boiler on the interior walls. U.S. Pat. No. 5,215,042 discloses a CFB reactor divided into compartments by at least one vertical, substantially gas-tight partition in the upper part of the combustion chamber. The partition wall comprises cooling tubes and is provided with at least one line with a distributing manifold to feed combustion air into the compartments.
U.S. Pat. No. 4,545,959 discloses a chamber for the treatment of particulate matter in a fluidized bed, comprising a duct with a triangular cross section on the bottom of the chamber, and an arrangement of holes or slots in each of the upwardly sloping side walls of the duct for directing an ancillary gas from the duct into the chamber.
The above-mentioned publications suggest introduction of gas into a reactor chamber, e.g., furnace chamber, through a partition wall within the chamber. A problem arises, however, as the ducting from the air or gas source chamber to the air or gas injection point may be rather long and cause a high pressure drop. A problem arises also in these conventional supply duct constructions due to solids backsifting, i.e., the problems with solid particles from the furnace tending to flow into the gas supply ducts and an increase in the pressure drop over the gas supply ducts. The increase in pressure drop may be very difficult to attend to or to take into consideration when controlling the gas supply.
Conventional bottom grid nozzle constructions, e.g., those equipped with bubble caps normally reaching upward from the bottom grid, would be exposed to heavy erosion if installed on a vertical partition wall within a fluidized bed, due to very high erosive forces caused by the downward flowing solid particle layers in the vicinity of the wall. In fluidized bed reactor furnaces, solid particles tend to flow upward in the middle of each furnace section and downward along its vertical side walls. Such downward flowing particles come in the lower part of the furnace sections, where the cross-sectional area of the furnace sections typically abruptly decreases, into intense turbulent motion which may locally lead to very strong erosive forces, e.g., also in the regions of secondary gas inlets. In the prior art, no special solution for preventing backsifting into gas nozzles or conduits arranged, for example, on furnace side walls, such as partition walls or exterior side walls has been disclosed.
It is, therefore, an object of the present invention to provide a fluidized bed reactor with a furnace construction having an improved gas supply configuration.
It is particularly an object of the present invention to provide an improved gas supply configuration suitable for large scale circulating fluidized bed (CFB) boilers.
It is, then, more specifically an object of the present invention to provide an improved secondary gas supply configuration arranged in an exterior side wall and/or a partition wall within the lower part of a furnace.
It is more specifically an object of the present invention to provide a fluidized bed reactor with improved gas supply means, with minimized backsifting of solid particles into gas supply conduits therein.
It is thereby also an object of the present invention to provide a fluidized bed reactor with improved gas supply means with decreased pressure losses in the gas supply means.
These and other objects of the present invention are achieved by providing a fluidized bed reactor that includes at least one furnace section delimited by sidewalls and a bottom grid, the at least one furnace section being provided for containing a bed of fluidized solid particles therein, and supplying means for introducing a gas into the at least one furnace section at a level above the bottom grid. The supplying means includes (i) a gas source chamber, (ii) at least one opening in at least one of the side walls at a level above the bottom grid, and (iii) at least one conduit, having a first end connected to the at least one opening at a first vertical level and a second end connected to the gas source chamber, for introducing gas from the gas source chamber to the at least one furnace section. The at least one conduit provides a solid flow preventing element for preventing solid particles from flowing backward from the at least one furnace section into the at least one conduit. As used herein, the term xe2x80x9csidewallsxe2x80x9d can refer to exterior side walls of the furnace and/or partition walls of the furnace, whether such partition walls are partial walls or complete walls.
In those large scale fluidized bed reactors to which the present invention can be applied, which are divided by dual-wall partitions into separate furnace sections, at least a portion of the free internal space between the partition walls may, according to one preferred embodiment of the present invention, constitute the gas source chamber or windbox, providing secondary or other gas to the furnace sections.
The gas source chamber may, on the other hand, if desired according to another preferred embodiment of the present invention be formed at another location, e.g., connected to an external side wall(s) or to the bottom grid.
Still further, the gas source chamber may be connected to at least one of the external side wall(s), the bottom grid and the partition walls (if such walls are so utilized).
Secondary gas or other similar gas is typically introduced into furnace sections through a plurality of gas injecting openings formed in the side walls delimiting the furnace sections. The openings may be arranged in a single row at the same vertical level in each wall, or the openings may, if desired, be arranged in some other configuration and at several different vertical levels in the walls. A conduit, such as a standpipe or a bent pipe construction, is according to the present invention disposed between each of the openings and the gas source chamber, for introducing gas from the gas source chamber through the openings into the furnace sections.
A solid flow seal is formed in the conduits so as to prevent solid particles from flowing backward into the conduit in a manner preventing or noticeably decreasing the introduction of gas from the gas source chamber to the furnace sections. Some minor back and forth flow of solid particles within the conduits close to the openings may be tolerable. The solid flow seals may be formed in different ways, e.g., depending on the location of the gas source chamber.
In a fluidized bed reactor in which the gas source chamber is formed in the space between two partition walls forming a partition on the bottom grid and/or in which the gas source chamber is attached to the external walls of the furnace, secondary gas/air nozzles or conduits in the form of open-ended standpipes may preferably be used. The standpipes may have a first open end connected to an opening in one of the partition walls and/or exterior side walls at a first vertical level l1, e.g., at the secondary air injection level, and a second open end opening into the gas source chamber at a second vertical level l2 which is at a higher level than the first vertical level. This construction may be used when at least a portion of the gas source chamber reaches to a vertical level above the injection level of the gas, e.g., the injection level of secondary air.
The standpipe preferably has a circular cross section, but other forms are possible, such as slot-like cross sections. The vertical extent of the standpipe, i.e., the difference l2xe2x88x92l1, has to be big enough to generally prevent solid particles from backsifting therethrough from the furnace section to the gas source chamber.
The standpipe may be bent at its lower end, such that the lower end thereof may be fastened more easily to a vertical or only slightly inclined side wall construction. The standpipe may even have a short, nearly horizontal lower portion in order to bring the standpipe out from the side wall construction. Preferably, a minimum distance or clearance is provided between the side wall and the standpipe along the entire length of the standpipe, i.e., also when the side wall is inclined and approaches the standpipe at the upper end thereof. Another solution would be to make the standpipe slightly inclined.
The standpipe is, however, preferably substantially upright, but may, due to constructional reasons and as discussed above, have a lowermost portion, forming a  less than 90xc2x0, typically about 45xc2x0, but always xe2x89xa730xc2x0 angle with the horizontal plane. The rest of the standpipe, i.e., the upper portion of the standpipe, is mainly upright forming a xe2x89xa730xc2x0 angle with the horizontal plane.
In a fluidized bed reactor having a gas source chamber at a substantially different location, e.g., partly or totally above or below the grid level, another conduit or nozzle construction may be used in order to bring up gas from the gas source chamber to, e.g., the secondary gas level. The conduit, which may be formed of a pipe or other similar element, has according to a preferred embodiment of the present invention the form of an upside down U-bend. A first end of the conduit is connected to an opening at a first vertical level l1 in one of the side walls and a second end of the conduit is connected at a third vertical level l3 to an opening in an enclosure delimiting the gas source chamber. The conduit has between its first and second ends an upward bent portion, having its highest point at a second vertical level l2, which is at a higher level than the first l1 and third l3 vertical levels. The first level, i.e., the secondary air injection level, typically is at a higher level than the third level, which may be, e.g., at the bottom grid level or below or above the grid level.
The vertical extent of an upright standpipe, or the height of the first portion of a bent conduit, correlates to the solid flow backsifting preventing ability of the conduit. The height difference xcex94l between the first l1 and second l2 vertical levels is directly related to the pressure required to move solid particles through the standpipe, e.g., the larger the xcex94l the longer the standpipe, and the less solid particles are able to backsift through the conduit.
Typically, a vertical column xcex94l of about 1.0 meter may be needed for providing an efficient solid flow seal against normal furnace pressure variations.
The constructions described above may be used, as discussed earlier, in fluidized bed reactors having the lower part of the furnace section divided by a dual-wall partition. Such a partition may, if desired, reach from the bottom grid up to the roof of the furnace, dividing the entire furnace chamber in two separate sections. Such furnace dividing walls preferably include at least one opening in their upper part to allow horizontal mixing of the gases and fluidized particles in the separate furnace sections.
The partition walls dividing the lower part of the furnace or the divisional walls dividing the entire furnace into two parts or sections may preferably be constructed of finned tube panels, where the flow direction of the cooling medium is upwards from a header on the level of or below the furnace bottom. The cooling tubes of a partition wall may extend substantially vertically up to the roof of the furnace thus forming a divisional wall within the furnace, the tubes providing an additional cooling surface area within the furnace.
In many known fluidized bed reactor constructions, the interior of the dual wall partitions contains various ducts for different purposes, but the interior space formed between the partition walls has not been otherwise utilized. When using, according to one aspect of the present invention, at least a portion of the interior of the dual wall partition as a gas source chamber such as a windbox for air or gas, which is to be distributed into the furnace above the primary air grid, space is correspondingly spared below the main furnace grid. Moreover, the required length of ducting between the windbox and air/gas introduction point in the furnace is minimized, which leads to decreased pressure losses, i.e., lower cost, compared to conventional constructions. The present invention then provides, due to the decreased pressure losses, a better air/gas distribution and hence, more optimal reaction conditions within the furnace. Also, by locating structures preventing backsifting of solid particles into the interior of a dual wall partition, the structures are protected from the erosive forces of moving solids in the vicinity of the partition.